Stamping curb link chain

Just finished adding a few extra links onto a customers curb link chain and they would like each link hallmarked to match the rest of the links. Does anyone have any advice on how to/what to use to brace the back of the link. Was planning on using a small dapping punch secured in a vice. just went to try it and discovered I need to grow an extra hand, one to hold the chain, one to hold the stamp and another to hit it with a hammer.
All advice welcome and appreciated,

If the marks are on the flats of the links, then shellac or another temporary fixative on a bench block should hold well enough for you to strike them successfully. Shellac is brittle, but if you’re only doing a few it should hold long enough.
Just make sure to press the chain well into the fixative so that it rests directly on the steel support. Also, don’t overheat the block when melting the shellac, you don’t want to draw the temper.

Great! Thanks Elliot, I’ll give it a go toda. Fingers crossed haha!

I used to use a pitch bowl for this, but pitch is always a messy cleanup.
Thermoplastic, sold under brand names such as Jet Set and Thermolock can be found in all the major jewelry tool suppliers. Look under Stone Setting supplies. Follow the directions for hot water or heat gun warming of the material to form and hold your piece. Stamp. Warm again to release.